Protactinium is a dense, silvery-gray actinide metal which readily reacts with oxygen and water vapor.
Core Principles & Analysis
Kasimir Fajans and Oswald Goehring first detected element 91 in 1913 as the short-lived isotope they called 'brevium'; Otto Hahn and Lise Meitner identified long-lived protactinium-231 in 1917-18.
Its name means 'parent of actinium', because protactinium-231, with a half-life of 32,760 years, alpha-decays directly into actinium-227.
In 1961 the UK Atomic Energy Authority processed about 60 tonnes of ore residue to extract just 125 grams of protactinium, long the world's main stock; the Pa-231/Th-230 ratio is now used to date ocean sediments.